This invention relates generally to a thermal type of flow rate sensor and, more particularly, to a thermal type of flow rate sensor designed to measure the quantity of intake air in internal combustion engines.
As known so far in the art, the flow rate of air in automotive internal combustion engines is measured with, e.g., a thermal type of flow rate sensor including a heat generator in an intake suction passage, which is kept at a certain temperature, for instance 100.degree. C. by current control, whereby the change in the amount of heat needed for the heat generator to be cooled is detected as a resistance change to determine the flow rate of air. In this regard, see "SENSOR TECHNOLOGY", September 1989, page 25. Such a flow rate sensor is controlled either at a constant temperature or with a constant current.
Such a thermal type of flow rate sensor, for instance, includes such an electric circuit as shown in FIG. 1. As illustrated, there are provided a heat-generating resistor R.sub.H and a temperature-compensating resistor R.sub.C which are in contact with the air within an air pipe 1, and resistors R.sub.1 and R.sub.2 are connected with the outside of an air passage 2 in the form of a bridge circuit. The heat-generating resistor R.sub.H is provided to measure the flow rate of air, while the temperature-compensating resistor R.sub.C is a resistor that is kept at the same temperature as that of the air in the air passage 2. For R.sub.H and R.sub.C, resistors having resistance values of 10-30.OMEGA. and 400-1000.OMEGA., respectively, are generally used. In FIG. 1, reference numeral 3 stands for a transistor; 4 a comparator; and 5 a terminal on which a sensor-driving voltage is impressed. While the bridge circuit is kept so well-balanced that the temperature of the resistor R.sub.H can be higher than the air temperature (i.e., the temperature of the resistor R.sub.C), a heating current is supplied to the bridge circuit to keep the temperature of the resistor R.sub.H constant. Then, an output 6 corresponding to the flow rate of air can be obtained in the form of a voltage across R.sub.1.
A problem with the conventional thermal type of flow rate sensor, however, is that it is difficult to detect the flow rate of air accurately, because the temperature-compensating resistor R.sub.C fails to make up for temperature accurately, resulting in an error in measuring the flow rate of air. One major reason would be that the electric circuit is designed without finding an accurate temperature coefficient of resistance of the heat-generating resistor of the conventional thermal type of flow rate sensor, as will be described later.
It is therefore an object of this invention to provide a thermal type of flow rate sensor which can dispense with not only such an electric circuit for temperature compensation as that used in the conventional thermal type of air flow-rate sensor but circuit adjustment as well and, nonetheless, can provide an accurate detection of the flow rate of a fluid over a wide temperature range.